1. Field of the Invention
The present invention relates to a DC power source unit that can be used for cordless electrically powered tools.
2. Description of the Related Art
Cordless electrically powered tools, such as electrical drills, can be used any place and are easy to use because no power source cables are required. However, the use of batteries invites some disadvantages. When the charge remaining in the batteries drops to less than an allowable limit, the batteries need to be replaced or recharged. Further, because the battery voltage gradually drops as the use of the battery progresses, the tool""s operation efficiency is not constant.
FIGS. 1 and 2 show a conventional DC power source unit suitable only for situations wherein the tool is used close to an AC power source and when work with the tool can be accomplished in a single location without the need to move the tool to different areas. The DC power source unit includes an AC/DC converting box 50, an input cable 57, and an output cable 54. The input cable 57 has one end connected to the converting box 50 and another end attached to a plug 56. The plug 56 is used for connection to an AC power source. The output cable 54 has one end connected to the converting box 50 and another end attached to an adapter plug 55. The adapter plug 55 is detachably connected to the electrically powered tool. As shown in FIG. 2, the converting box 50 includes a transformer 51 for dropping the AC 100 volts to ten and several volts, a rectifier 52 for rectifying the AC voltage to a DC voltage, and a smoothing capacitor 53 for smoothing the waveform of the rectified voltage.
With the DC power source unit shown in FIGS. 1 and 2, the AC voltage supplied from a commercial outlet is converted to a DC voltage to be supplied to the tool. Such a DC power source unit allows the continuous use of tools, thereby ensuring constant work efficiency.
The motors contained in electrically powered tools do not heat up significantly when the tool is driven with batteries. For this reason, the battery powered tools are, generally, not provided with motor cooling fans. Even if the fans are provided, they are small size fans.
When a tool is driven using the conventional DC power source unit, the tool tends to be used continuously for a long period of time, causing the temperature of the motor to rise significantly. This shortens the service life of the motor or the tool. To overcome the above problem, one solution would be providing a large-size cooling fan within the electrically powered tool. However, a large-size cooling fan would increase the overall size of the tool. Large size tools are not desirable for the operator to handle or operate. Another solution to this problem would be providing a temperature sensor in the tool to monitor an abnormal temperature rise within the tool. Power supply to the motor can be controlled in response to the output of the sensor. However, it is difficult to add a new component to the electrical circuitry of the tool and to modify control circuitry of the tool. Even if such addition and modification of the control circuit were made, the resultant electrical arrangement would be complicated.
Some conventional power tools are provided with an internal DC power supply unit. However, the power tools must be made in a large size to accommodate the internal DC power supply unit. Also, the power tools are heavy because the internal DC power supply unit adds to the overall weight of the power tools. Further, because the DC power supply unit is provided internal to a particular power tool, that DC power supply unit can not be used for any other power tools.
In view of the foregoing, it is an object of the present invention to provide an improved DC power source unit that is suitable for use in electrically powered tools.
Another object of the present invention is to provide a DC power source unit that minimizes shortening of the service life of electrically powered tools.
Still another objective of the present invention is to provide a DC power supply unit that is easy to use, and that can be easily used with a great variety of power tools for supplying power to the power tools.
To achieve the above and other objects, several types of DC power source units are provided for supplying DC voltage to an electrically powered tool.
According to one aspect of the present invention, a DC power source unit for supplying DC voltage to an electrically powered tool, includes a main unit, an adapter plug, an output cable, and output voltage switching means. The main unit has an output and produces DC voltage from the output. The adapter plug has a fitting portion adapted for electrically coupling with the electrically powered tool. The output cable is attached, either integrally or detachably as described below, at one end to the main unit and at another end to the adapter plug. The output voltage switching means is for switching a level of voltage supplied by the main unit to the electrically powered tool, to a level that matches rated voltage of the electrically powered tool that corresponds to the fitting portion of the adapter plug.
A battery pack can be provided for supplying DC voltage to the electrically powered tool. In this case, it is desirable that the battery pack include a fitting portion that is shaped substantially the same as the fitting portion of the adapter plug.
The output cable can be either integrally connected to or detachably engaged with the adapter plug. When integrally connected, it is desirable that the output cable be detachably engaged with the main unit. In this case, a connector that is integrally connected to the output cable is provided for detachably connecting the output cable to the main unit. A voltage setting means is housed in the connector. The voltage setting means corresponds to the rated voltage of the electrically powered tool that corresponds to the adapter plug integrally connected to the other end of the output cable. The output voltage switching means switches the level of supplied voltage according to the voltage setting means, upon connection of the connector to the main unit.
When the output cable is detachably engaged with the adapter plug, it is desirable that the adapter plug house therein a voltage setting means corresponding to the rated voltage of an electrically powered tool that corresponds to the adapter plug. In this case, the output voltage switching means switches the level of voltage according to the voltage setting means, upon coupling of the adapter plug to the electrically powered tool.
A plurality of adapter plugs can be provided in a one-to-one correspondence with a plurality of electrically powered tools. In this case, each adapter plug is shaped to enable coupling with a corresponding electrically powered tool that has a corresponding rated voltage, and is also shaped to prevent coupling with all non-corresponding electrically powered tools that have a non-corresponding rated voltage. Also, a connector is provided for detachably connecting the cable to each adapter plug. The connector is adapted to connect to all adapter plugs regardless of the rated voltage of the corresponding electrically powered tool. It is particularly beneficial in this case, that each adapter plug house therein a corresponding voltage setting means as described above.
According to another aspect of the invention, a DC power source unit includes a main unit that has an output and produces DC voltage from the output, an adapter plug that is electrically coupled to the electrically powered tool in use, a connector that is electrically connected to the output of the main unit, and an output cable having one end attached to the adapter plug and another end attached to the connector. An output voltage setting means is also provided in the adapter plug for setting a level of voltage to be supplied to the electrically powered tool. The level of voltage set by the output voltage setting means corresponds to a rated voltage of the electrically powered tool. A load current detection means is also provided for detecting a load current flowing in the electrically powered tool and outputs a detection signal indicative of the load current. An output voltage changeover means is also provided for changing over the level of voltage to be supplied to the electrically powered tool in response to the level of voltage set by the output voltage setting means. A control means is provided for transmitting the level of the voltage set by the output voltage setting means to the output voltage changeover means based on the detection signal output from the load current detection means. The adapter plug may be detachably connected to the one end of the output cable. Preferably, the output voltage setting means is a resistor.